The invention relates to a method for controlling the dewatering profile in a former of a paper or board machine, in which method water is removed from a web between two forming wire loops, inside which forming wire loops dewatering elements have been placed of which at least one comprises a blade element which extends transversely across the web with respect to the running direction of the wires.
The invention also relates to a regulation arrangement for controlling the dewatering profile in a former of a paper or board machine, which former comprises two forming wire loops having dewatering elements placed inside them, of which elements at least one comprises a blade element which extends transversely across the web with respect to the running direction of the wires and is in contact with one of the forming wires.
Dewatering in the drainage area of a twin-wire former depends, among other things, on the quality of wires, the dewatering elements used, dewatering pressure (tension of wires on curved surfaces) as well as on drainage distance. As dewatering elements are generally used, among other things, forming rolls, forming shoes and dewatering boxes, the dewatering effect of which can be further enhanced by means of blade elements placed on the opposite side of the web, the function of which blade elements can be to doctor water from the surface of the wire, generate pressure pulses in the web which is being formed and/or guide the run of the web. If the flow channel defined between the surface of a dewatering element and a blade element placed on the opposite side of the web is filled with water and if said elements are not totally parallel over their entire length, the thickness of the stock suspension layer is not uniform at them across the entire width of the web. In that connection, lateral flows readily occur in the stock, which flows may cause problems in various property profiles of the paper that is being manufactured. Initially, attempts are made to mount the dewatering and blade elements straight, but measurements or regulations are generally not performed any more during running of the paper machine. The problems associated with the dewatering profile are aggravated when the former is run with large flow volumes and there are dewatering elements on both sides of the web. As a result, there may be problems both in the basis weight profile of paper and in its fibre orientation profile. In practice, it is very difficult to distinguish whether the cause of the problems is the headbox or the dewatering elements disposed after it.
In order to achieve good formation and to control the basis weight, moisture content and dewatering in the cross direction of the web, it would be important to know the liquid amount between the wires and its flow rate profile in the web former. The thickness profile of the stock flow in the former cannot be directly measured by the present methods.
FI patents 92940 and 103995 describe regulation arrangements for controlling cross-direction property profiles of a paper web manufactured on a twin-wire former. Water is removed from the paper web after a forming gap under compression between two forming wires utilising dewatering and/or forming ribs or blades whose deflection is regulated in the cross direction of the machine, thereby affecting the cross-direction retention profile of the web. In the method, a cross-direction property profile, such as the fibre orientation profile, filler profile, basis weight profile and/or slice profile of the paper web being manufactured is/are measured, and the deflection of loading blades is regulated based on the measurement signals. Thus, the actual thickness profile of the stock flow at the blades is not monitored in the method, but the regulation of the deflection of the blades is based on control signals derived from elsewhere.
U.S. Pat. No. 5,530,346 describes a method and a device for contact-free measurement of the size and shape of a slot, and it is mentioned that one application of the method is measurement of the height and profile of a headbox slice. A current conductor is arranged at the edge of the slot to be measured, to which conductor an alternating current of low frequency is passed. The conductor produces a magnetic field around it and the strength of the magnetic field is measured by means of magnetometers attached to the opposite edge of the slot. The height and the shape of the slot are calculated based on the strength of the magnetic field. The document describes a mere measurement method but not the use of the measurement result as a control signal.